High vacuum system method and apparatus



l 1966 H. G. NGLLER ETAL 3, 3

HIGH VACUUM SYSTEM METHOD AND APPARATUS Filed May 7, 1964 DISTRIBUTION 37 VALVE SERVO MECHANISM CONTROL AMPLIFIER fi'weizfofls: Hans Georg H6303, Weflne? $.Biiciz3er,

6 /v as-[away United States Patent L 45,0 16 Claims. (Cl. 23t 101) This invention relates to a high vacuum system apparatus and method of operation. More particularly the invention relates to high vacuum system apparatus and an operational method therefor which utlilize diffusion vacuum pumps.

Important considerations in most high vacuum applications are gas throughput capacity and ultimate pressure. Gas throughput capacity is the quantity rate at which gas can be pumped by the pumping system and determines the time required to evacuate a given vacuum system to a desired pressure. This factor is particularly important in those cyclical vacuum applications in which a relatively large number of system pump down cycles are required. In such applications the time required for each individual pump down cycle can have a significant effect on the efficiency of the particular vacuum process being performed. Gas throughput capacity of a pumping system also determines the pressure which can be maintained by the system when pumping against a given constant gas load with higher throughput capacities providing lower operating pressures. In either of these application the throughput capacity of the pumping system is important only if a sizeable quantity of gas is to be pumped. Such gas quantities can come from either the inherent wall outgasing of systems having a significant volume or the substantial load provided by gas producing process applications.

The ultimate pressure of a given vacuum system is the lowest pressure which can be attained in that system. One factor affecting the ultimate pressure in the systems which utilize diffusion vacuum pumps is the high vapor pressure components existing in the pumping fluid of the diffusion pump. These components exist as original impurities, pumping fluid decomposition products, etc. and are able during the pumping process to diffuse from the pump into the vacuum system thereby greatly limiting the ultimate vacuum attainable. This problem is present even when extremely pure, carefully distilled pumping fluids are used.

A method of substantially reducing this effect in diffu sion vacuum pumps was disclosed in a paper by H. G. Noller, G. Reich, and W. Bachler in the 1957 Vacuum Technology Transactions, pages 6-12. The test results presented in this paper show that the elimination of cooling from that portion of the diflusion pump wall which intercepts a vapor stream from the bottom jet of a conventional multi jet diffusion pump will greatly improve the ultimate pressure attainable in a given system. The improved performance is attributed to a degassing action which takes place on the pumping fluid. As the vapor stream leaving the bottom jet strikes the hot pump wall, the volatile components are driven out of the pumping fluid and are removed from the pump by the mechanical forepump.

It is therefore an object of this invention to provide a diffusion vacuum pump system of substantial volume which can reach a low ultimate pressure in a relatively short pump down time.

Another object of the invention is to provide a diffusion vacuum pump operating method which can produce a low ultimate pressure after pumping a substantial gas quantity in a relatively short period of time.

One feature of this invention is the provision of a high vacuum system having a diffusion vacuum pump with a primary cooling coil for that portion of the pump wall which intercepts the vapor stream from the upper jets of a conventional vertically stacked multi jet assembly and an independent auxiliary cooling coil for that portion of the pump wall which intercepts the vapor stream from the bottom jet of the pump and a vacuum chamber having a volume magnitude in cubic units which is at least eight times the magnitude of the cross sectional area in square units of the diffusion pump inlet opening.

Another feature of this invention is the provision of a high vacuum system of the above featured type wherein the auxiliary cooling coil also directly cools an exhaust tubulation of the diffusion vacuum pump.

Another feature of this invention is the provision of high vacuum pumping systems of the above featured types including automatic pressure responsive control devices for regulating the flow of cooling fluid in the primary and auxiliary cooling coils.

Another feature of this invention is the provision of a high vacuum system of the above featured type wherein the pressure responsive control device is positioned so as to be responsive to the pressure in the vacuum chamber.

Another feature of this invention is the provision of a method for operating a vacuum system of the above featured types wherein both primary and auxiliary cooling coils are utilized to provide high diffusion pump throughput during periods when the pressure in the vacuum chamber is above 10* mm. Hg and discontinuing the use of the auxiliary cooling coil to provide good degassing of the diffusion pumping fluid during periods when the pres sure in the vacuum chamber is below lO -S mm. Hg thereby permitting the attainment of low ultimate pressure in the vacuum chamber.

Another feature of this invention is the provision of an operating method for a vacuum system of the above featured types wherein both primary and auxiliary cooling coils of the diffusion pump are utilized to provide a high pumping throughput while a certain minimum quantity of gas is pumped from the vacuum chamber, the magnitude of this certain gas quantity in Torr cubic feet being at least 30 times the magnitude of the cross sectional area of the diffusion pump inlet port in square feet, and discontinuing the use of the auxiliary cooling coil while pumping an additional quantity of gas from the vacuum chamber thereby permitting the attainment of a low ultimate pressure therein.

These and other features and objects of the present invention will become apparent upon a perusal of the following specification taken in conjunction with the accompanying drawing which provides a schematic illustration of 'the high vacuum system apparatus according to the present invention.

Referring to the drawing there is shown a diffusion vacuum pump 11 including a cyclindrical pump hosing 12 which encloses a plurality of vertically stacked pumping jet assemblies 13. The upper end of the pump housing 12 is open to form the inlet port 14 and the lower end is closed to form the boiler section 15. Immediately above the boiler section 15 in the lower portion of the pump housing 12 is an exhaust port 16 which opens into the exhaust tubulation 17. Attached to the outer wall of the pump housing 12. is the primary cooling coil 18 having a large number of turns which cool the inner surface of the pump housing 12. The top turn of primary coil 18 is positioned above the lip of top jet assembly 19 and the bottom turn thereof is horizontally aligned with the lip of bottom jet assembly 21. Also positioned on the outer walls of the housing 12 is the auxiliary cooling coil 22 which includes a plurality of turns positioned below the lip of bottom jet assembly 21 and an additional plurality of turns surrounding the exhaust tubulation 17.

The cylindrical connecting tubulation 23 joins the diffusion vacuum pump 11 to the cylindrical vacuum chamber 24 via the connecting flange assemblies 25 and 26. Positioned in the upper portion of the pump housing 12 near the inlet 14 is the pressure gauge 31 which is connected to the control amplifier 32 by the electrical lead 33. The output of the control amplifier 32 operates the servo-mechanism 34- which in turn controls the cooling fluid distribution valve 35. The distribution valve 35 is equipped with three pairs of inlet, outlet tabulations. The first pair 36 is adapted for connection to a source of cooling water (not shown). The second pair 37 is connected to inlet port 38 and outlet port 39 of the primary cooling coil 18. The third pair 41 is connected to the inlet port 42 and the outlet port 43 of the auxiliary cooling coil 22. The distribution valve 35 is a four position valve adapted to supply cooling fiuid only to tubulation pair 37, only to tubulation pair 41, to both pairs simultaneously, or to neither pair.

During operation of the vacuum system according to the present invention, cooling water is circulated to both primary and auxiliary coils 18 and 22 during periods when high throughput pumping performance is desired and only through primary coil 18 during periods when a low ultimate pressure is to be attained. As fully described in the :above noted technical paper, the provision in a diffusion pump of a hot surface to intercept the jet stream from the bottom jet in a vertically stacked multi jet assembly produces a thorough degassing of the pumping fluid allowing the attainment of a much lower ultimate pressure.

However, tests have now shown that the use of cooled walls to intercept the vapor stream from the bottom jet in a vertically stacked array will increase the gas throughput of a given pump by about 20% in the high gas load pressure region between 10 and 10- mm. Hg. This increase in gas throughput is even more significant than is at first apparent, especially in those vacuum applications which provide a constant gas load in the critical 10 and 10 mm. Hg pressure range. In such applications the 20% increase in the throughput will permit a given pumping system to maintain an equilibrium system pressure of about /6 the equilibrium pressure previously attainable with a given constant gas load.

Thus the present invention provides a diffusion pump vacuum system which can function in two distinct operational modes, -i.e. high throughput and good degassing modes. The great advantage of the invention exists in systems where the quantity of gas to be pumped is relatively large compared to the pumping speed of the particular diffusion vacuum pump utilized.

Such gas loads will normally come from one of two possible sources. The first source is the contaminated walls of the vacuum system itself which during pump down will continually outgas providing a large quantity of gas to be pumped. Another source of gas in a vacuum system can be a gas producing process which is taking place in the vacuum system such as, for example only, the flashing of cathodes in vacuum tube manufacture, vacuum degassing processes, etc. In either instance the vacuum system being evacuated must be relatively large compared to the pumping speed of the diffusion vacuum pump if the gas load is to be significant. This is because in the first instance above the area of the outgassing system walls supplying the gas load must be relatively large or in the second case above the vacuum system must be relatively large to accommodate the gas producing process being performed.

Therefore the high vacuum system of the present inven tion is uniquely suited to systems in which the volume magnitude of the vacuum chamber to be evacuated is at least eight times the cross sectional area magnitude of the diffusion vacuum pump inlet port where the cubic units of the vacuum chamber volume and the square units of the inlet port cross sectional area are the same. The cross sectional area of the diffusion pump inlet port is chosen for this comparison because the relative pumping speed of conventional diffusion vacuum pumps is substantially proportional to this inlet port area and because there are no accepted methods for detenmining absolute pumping speeds.

In a preferred operational method of the present invention, the diffusion pump 11 begins to evacuate the chamber 24 with the distribution valve 35 positioned so as to provide a flow of cooling water in both the primary coil 18 and the axuiliary coil 22. During this period of operation the pump will operate in its high throughput mode since substantially all the inner wall surface is being cooled. The auxiliary coil 22 will also serve to cool the exhaust tubulation 17 which will also substantially reduce oil losses during this period. The pumping fluid entrained in the gas exhaust will be condensed on the cold inner walls of the tubulation 17 and run back into the boiler section 15 for additional use rather than being pumped completely out of the diffusion pump by a conventional mechanical forepump (not shown) which is connected for gas communication with the exhaust tubulation 17.

The gauge 31 which can be, for example only, a therrnocouple gauge responds to the pump inlet pressure to produce an output signal proportional to the pressure in the vacuum chamber 24. When the pressure in the chamber 24 reaches about 10 mm. Hg, the magnitude of this output signal will cause the amplifier 32 to exert a control function on the servomechanism 34 which will in turn perform a mechanical operation on the distribution valve 35. This operation will close the inlet-outlet tubes 41 discontinuing the supply of cooling water to auxiliary coil 22 while the inlet-outlet tubes 37 remains open continuing to supply cooling water to the primary coil 18. As a result, the inner pump wall adjacent to the bottom jet assembly 21 and the exhaust tubulation 17 will be warmed by the boiler section 15 causing the pump to operate in its good degassing mode. The pump will then be effective to produce a lower ultimate pressure in the vacuum chamber 24. If the pressure in the chamber should again rise above 10 millimeters of mercury, the resultant output signal of the pressure gauge 31 will cause a reverse control operation on the servomechanism 34 and the distribution valve 35 to again open the inlet-outlet tubes 41. The circulation of cooling water through the auxiliary coil 22 will again provide the high throughput mode of operation.

It is preferred that the change in operating modes take place at about 10 millimeters of mercury because upon reaching this pressure a substantial quantity of the gas load to be pumped will already have been removed from the chamber 24. Also it is desirable for the pump to operate in the good degassing mode during as much of its total operating time as possible since there is a certain time lag in obtaining totally degassed pumping fluid. This is because only that portion of the pumping fluid which strikes the warm inner surfaces of the housing 12 will become degassed. Therefore sufficient time to allow thorough circulation of the pumping fluid is required to obtain complete degassing. Conversely the change to the high throughput mode takes place very quickly requiring only that time necessary for the auxiliary coils 22 to cool the inner surface of the housing 12 adjacent the bottom jet assembly 21. i

As noted above the great advantage of the present invention exists where the quantity of gas to be pumped is relatively large compared to the pumping speed of the particular diffusion vacuum pump utilized. Also the major portion of the gas quantity should be pumped while the pump is operating in the high throughput mode if the mode switching is to be particularly advantageous. Therefore another preferred method of operation accord ing to the present invention consists of pumping a certain minimum quantity of gas while the diffusion pump operates in the high throughput mode and then switching to the good degassing mode to pump an additional quantity of gas. Preferably the magnitude of the certain quantity of gas in Torr cubic feet should be at least 30 times the magnitude of the cross sectional area in square feet of the inlet port for the diffusion pump being used. As discussed above this area is generally proportional to the pumping speed of the pump. This ratio of gas quantity to pump inlet size provides for a quantity of gas which can be pumped in the region of pressure between 1() mm. Hg by representative commercially available diffusion pumps in a time of approximately one minute. It is believed that for pumping operations which require less than one minute in that pressure range, the switching of modes would not be advantageous because the pumping time saved would be small. Also the transient time required for changing from one mode to another would be substantial compared to the pumping time in the high throughput pressure region. This preferred method of operation is particularly advantageous in periodic processing applications such as, for example only, the processing of vacuum tubes in which cathode flashing operations produce relatively large periodic gas bursts.

Although the particular embodiment and methods shown and discussed are preferred, other methods and configurations could also prove effective. For example only, a plurality of diffusion vacuum pumps could be connected to a single vacuum chamber. In this case the inlet areas used in the above formulas would of course comprise the total inlet port area for all the diffusion vacuum pumps being used. It is therefore intended in the following claims that the recitation of a diffusion vacuum pump is meant to include a plurality of diffusion vacuum pumps and in such case the inlet area recited is meant to include the total inlet port area of all diffusion vacuum pumps directly connected to the vacuum chamher.

What is claimed is:

1. A high vacuum apparatus comprising in combination:

(a) a diffusion vacuum pump comprising a pump housing, a boiler section in said pump housing and adapted to heat .a pumping fluid, an inlet and an exhaust port in said pump housing, a plurality of vertically stacked pumping jet assemblies positioned in said pump housing above said boiler section, said jet assemblies adapted to accept a vapor stream evaporating from said boiler section and to direct it in a downward direction, an inner wall in said pump housing adapted to intercept the directed vapor stream from said jet assemblies, an auxiliary cooling means in heat exchanging relationship with said pump housing and adapted to cool that portion of said inner wall which intercepts the vapor stream directed from the lowest one of said jet assemblies, a primary cooling means in heat exchanging relationship with said pump housing and adapted to cool that portion of said inner wall which intercepts the vapor stream directed by at least one upper one of said jet assemblies, and means for independently controlling said primary and auxiliary cooling means;

(b) a vacuum chamber having a volume whose magnitude is at least eight times the magnitude of the cross sectional area of said inlet port of said pump housing where the cubic units of the vacuum chamber volume are the same as the square units of the inlet port cross sectional area; and

(c) connection means connecting said inlet port of said pump housing with said vacuum chamber so as to provide for gas communication therebetween.

2. The apparatus according to claim 1 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is adapted to directly cool said outlet tubulation.

3. The apparatus according to claim 1 wherein said primary and auxiliary cooling means comprise tubulation means in contact with said pump housing and adapted to circulate a cooling fluid medium.

4. The apparatus according to claim 3 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is in contact with said exhaust tubulation so as to provide direct cooling thereof.

5. The apparatus according to claim 1 including regulation means for regulating said primary and auxiliary cooling means independently of each other and pressure responsive means operatively connected so as to control said regulation means.

6. The apparatus according to claim 5 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is adapted to directly cool said outlet tubulation.

7. The apparatus according to claim 5 wherein said primary and auxiliary cooling means comprise tubulation means in contact with said pump housing and adapted to circulate a cooling fluid medium.

8. The apparatus according to claim. 7 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is in contact with said exhaust tubulation so as to provide direct cooling thereof.

9. The apparatus according to claim 5 wherein said pressure responsive means is connected. so as to be responsive to the pressure existing in said vacuum chamber.

10. The apparatus according to claim 9 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is adapted to directly cool said outlet tubulation.

11. The apparatus according to claim 9 wherein said primary and auxiliary cooling means comprise tubulation means in contact with said pump housing and adapted to circulate a cooling fluid medium.

12. The apparatus according to claim 11 including an outlet tubulation connected to said pump housing so as to receive the exhaust gas from said exhaust port and wherein said auxiliary cooling means is in contact with said exhaust tubulation so as to provide direct cooling thereof.

13. A method of operating a vacuum diffusion pump which is connected to evacuate a vacuum chamber and is of the type having a pump housing with inlet and exhaust ports, a plurality of vertically stacked pumping jet assemblies, an inner wall portion positioned to intercept a vapor stream directed by the bottom jet assembly and an inner wall portion positioned to intercept a vapor stream from at least one upper jet assembly which method comprises the steps of directly cooling both inner wall portions during periods when the pressure in the vacuum chamber is above a certain pressure of about 1() mm. Hg and discontinuing direct cooling of the inner wall portion positioned to intercept a vapor stream directed by the bottom jet assembly during periods when the pressure in the vacuum chamber is below a certain pressure of about 1() mm. Hg.

14. The method according to claim 13 wherein the diffusion vacuum pump is of the type having an exhaust tubulation and including the steps of cooling the exhaust tubulation during periods when the pressure in the vacuum chamber is above said certain pressure of about 10 mm. Hg and discontinuing cooling of the exhaust tubulation during periods when the pressure in the vacuum chamber is below said certain pressure of about 10- mm. Hg.

15. A method of operating a vacuum diffusion pump which is connected to evacuate a vacuum chamber and is of the type having a pump housing with inlet and exhaust ports, a plurality of vertically stacked pumping jet assemblies, an inner wall portion positioned to intercept a vapor stream directed by the bottom jet assembly and an inner wall portion positioned to intercept a vapor stream from at least one upper jet assembly which method comprises the steps of pumping out of the vacuum chamher a certain quantity of gas with the ditfusion pump while directly cooling the inner wall portion positioned to intercept a vapor stream directed by the bottom jet assembly, the magnitude of said certain quantity of gas in Torr cubic feet being at least 30 times the magnitude of the cross sectional area of the diffusion pump inlet port in square feet and discontinuing the cooling of the inner Wall portion positioned to intercept a vapor stream from '8 the bottom jet assembly while pumping from the vacuum chamber with the diffusion pump an additional quantity of gas.

16. The method according to claim 15 wherein said diffusion vacuum pump is of the type having 'an exhaust tubulati'on and including the steps of directly cooling the exhaust tubulation while pumping said certain quantity of gas and discontinuing the cooling of the exhaust tubulation while pumping said additional quantity of gas.

References Cited by the Examiner UNITED STATES PATENTS 2,630,266 3/1953 Lawrence 230-101 MARK NEWMAN, Primary Examiner. WARREN E. COLEMAN, Examiner. 

1. A HIGH VACUUM APPARATUS COMPRISING IN COMBINATION: (A) A DIFFUSION VACUUM PUMP COMPRISING A PUMP HOUSING, A BOILER SECTION IN SAID PUMP HOUSING AND ADAPTED TO HEAT A PUMPING FLUID, AN INLET AND AN EXHAUST PORT IN SAID PUMP HOUSING, A PLURALITY OF VERTICALLY STACKED PUMPING JET ASSEMBLIES POSITIONED IN SAID PUMP HOUSING ABOVE SAID BOILER SECTION, SAID JET ASSEMBLIES ADAPTED TO ACCEPT A VAPOR STREAM EVAPORATING FROM SAID BOILER SECTON AND TO DIRECT IT IN A DOWNWARD DIRECTION, AN INNER WALL IN SAID PUMP HOUSING ADAPTED TO INTERCEPT THE DIRECTED UAPOR STREAM FROM SAID JET ASSEMBLIES, AN AUXILIARY COOLING MEANS IN HEAT EXCHANGING RELATIONSHIP WITH SAID PUMP HOUSING SAID ADAPTED TO COOL THAT PORTION OF SAID INNER WALL WHICH INTERCEPTS THE VAPOR STREAM DIRECTED FROM THE LOWEST ONE OF SAID JET ASSEMBLIES, 